Physics for Students of Science and

Engineering

PART II: ELECTRONS AND PHOTONS

Summer Semester 2015

Victor Karpov

PHYS 2140

Syllabus

1. Check out the syllabus carefully: the schedule

of lectures, quizzes and exams, the grading

system, and the homework.

2. This is all also on the Physics 2140 web site.

3. Also be sure you know the time and place of

your lab section.

4. Buy lab manual from UT store

Web site

<http://astro1.panet.utoledo.edu/~vkarpov/Physics-2140.html>

The Daily Quizzes

• Similar system as used in 2130.

• Daily quizzes... sometime

• Encourage attendance, interaction,

feedback.

• Research shows these things are good!

Homework

• For tomorrow:

– Ch. 21 Questions 1,2; Problems

2,7,13,65

What is physics?

• The structure of the physical world.

– What basic units are things made of?

– The fundamental particles

• Quarks, leptons, gluons, photons, gravitons

• The laws of nature.

– How do these basic units behave?

– The fundamental interactions

• Gravitational, Electromagnetic, Nuclear

This Semester: 4-week segments

1. Electric Charges and Fields

Chs. 21-25

2. Electrodynamics

Chs 26-32

3. Optics and Relativity

Chs 33-37

4. Quantum Physics

Chs 38-44

A World of Electric Charge

• The world is made of charged particles

• The common ones are

– The proton: Charge Qp = +e

– The electron: Charge Qe = e

– The neutron: Charge Qn = 0

• There is also the photon:

– Photons have zero charge, but they

interact with the charges of the others.

Atoms

Electrons

Nucleus

Protons

Neutrons

Atomic number and mass number

• Z = atomic number = no. of e’s = no. of p’s(so Q=0 for a neutral atom)

• A = mass number= no. of protons + no. of neutrons(mp = mn >> me)

• NA = Avagadro’s number = 6 1023

(number of atoms in one mole )

NA is defined so that the mass

of NA atoms is A grams

Example Two isotopes of uranium:

U235 ( 235U92 ) has Z=92, A=235

(92p’s, 92e’s, 143n’s)

U238 ( 238U92 ) has Z=92, A=238

(92p’s, 92e’s, 146n’s)

Question: How many atoms in 1 kg of U235?

Answer: One mole is 235 grams, so 1 kg is

1000/235 = 4.26 moles. The number of atoms in a

mole is NA = 6 x 1023 so the answer is

2423106.210626.4 atoms

Surface of a pure metal

showing individual atoms.

Chapter 21

• Electric Charge

• Coulomb’s Law

• Review

– Powers of Ten

– Vector Calculations

– Atoms and Electrons

I. Electric Charge

• Two kinds: positive and negative

• Matter is made of charged particles:

protons, electrons, atoms, molecules

• Charge is conserved and quantized

• The elementary charge: e = 1.6 x 10-19 C.

• Electric current -- the rate of flow of charge

• Conducting and insulating materials

II. Coulomb’s Law

• Inverse square law, attraction and repulsion

• SI units: Coulomb and Ampere

• The Coulomb constant: k = 9 x 109 SI units.

2

21

r

QQkF

III. Vector notation

rFF

rF

r

rr

rF

ˆ

ˆ||

runit vecto ˆ

||

r̂

r

F

If two forces act on a body, then the net force is the

vector sum:

BUT remember that the magnitudes may NOT add:

does not mean that A = B + C

III. Vector Notation

• Read Chapter 21

• Note Checkpoints 3,4

• Do Questions 1,2 – not graded

• Do Problems 2, 7, 13, 65

Chapter 21 Homework

Syllabus

• Does everyone have a syllabus?

• Do you know the time and place of your

lab section?

• Any questions about the course?

Notes about quizzes and exams

• Do the powers of 10 in your head.

• Use only one or two significant figures –

that’s enough to show you have the basic

principles right.

109 10109 kFor example SI units

Example ProblemTwo charges are separated by 3 meters. If

each charge is 2 microcoulombs, what is the

force by one charge on the other?

Solution

CCQ6

1022

2

269

2

2

3

)102(109

r

QkF

NF3

3

1049

1049

Chapter 21: Charge and

Coulomb’s Law

If you don’t yet have syllabus and homework

schedules please pick them up now.

Better find on the web

• Find 2140 homepage (physics.utoledo.edu)

• Read Chapter 21

• Note Checkpoints 3,4

• Do Questions 1,2

• Do Problems 2, 7, 13, 65

Chapter 21 Homework

I. Electric Charge

• Two kinds: positive and negative

• Matter is made of charged particles:

protons, electrons, atoms, molecules

• Charge is conserved and quantized

• The elementary charge: e = 1.6 x 10-19 C.

• Electric current -- the rate of flow of charge

• Conducting and insulating materials

II. Coulomb’s Law

• Inverse square law, attraction and repulsion

• SI units: Coulomb and Ampere

• The Coulomb constant: k = 9 x 109 SI units.

2

21

r

QQkF

If two forces act on a body, then the net force is the

vector sum:

BUT remember that the magnitudes may NOT add:

does not mean that A = B + C

III. Vector Notation

Notes about quizzes and exams

• Do the powers of 10 in your head.

• Use only one or two significant figures –

that’s enough to show you have the basic

principles right.

9109kFor example SI units

The quantum of charge

The world is made of atoms, which are made

of protons, neutrons and electrons.

• Proton has charge +e.

• Electron has charge –e.

• Neutron has charge 0.

Fundamental quantum of charge: e = 1.60 10-19 C

Atom is mostly empty space.

Tiny nucleus contains protons, neutrons.

Moving Charge

• I can charge an object by adding or removing electrons.

• When I comb my cat, I move electrons from the fur to the rubber comb, leaving the cat with a net positive charge, and the comb with a negative charge.

• Charge conservation means that, if both cat and comb were originally neutral, then

0 combcat QQ

Atomic number and mass number

• Z = atomic number = no. of e’s = no. of p’s(so Q=0 for a neutral atom)

• A = mass number= no. of protons + no. of neutrons(mp = mn >> me)

• NA = Avagadro’s number = 6 1023

(number of atoms in one mole )

NA is defined so that the mass

of NA atoms is A grams

Example Two isotopes of uranium:

U235 ( 235U92 ) has Z=92, A=235

(92p’s, 92e’s, 143n’s)

U238 ( 238U92 ) has Z=92, A=238

(92p’s, 92e’s, 146n’s)

Question: How many atoms in 1 kg of U235?

Answer: One mole is 235 grams, so 1 kg is

1000/235 = 4.26 moles. The number of atoms in a

mole is NA = 6 x 1023 so the answer is

2423106.210626.4 atoms

Another example

• Estimate the force on a person who lost

electrons from 1 gram of his/her body?

• Model body with a ~1 m3 water sphere,

• 1 mole of H20 is 18g.

• 1 g has n=NA/18 ~ 1022 atoms

• Electric charge Q=ne ~ 103 C

• Force F=kQ2/r2 ~ 1010 x (103)2 / 12=1016 N

F1m

Huge! Electrostatic forces are strong.

Coulomb’s Law:

Action at a Distance?

• One charged object exerts a force on another.

• Like charges repel, unlike charges attract.

• How can a force be exerted at a distance?

• Next chapter: the electric field.

The inverse square law

• True for both electricity and gravity! Why?

• Because space is three-dimensional.

• F 1/r2

• If r 2r then F F/4

• The force is inversely proportional to the

square of the distance

The Coulomb Law Constants

Coulomb’s experiment gives F = kQ1Q2/R2

and determines the electrostatic constant

k = 8.99109 9109 SI units

This is often written as 04

1

k

with the permittivity constant having the value

12

0 1085.8 SI units

Example 1Two charges are separated by 3 meters. If

each charge is 2 microcoulombs, what is

the force by one charge on the other?

Solution

CCQ6

1022

2

269

2

2

3

)102(109

r

QkF

NF3

3

1049

1049

Two charges are separated by 2 m and repel each

other with a force of 20 N. If they are moved to a

separation of 4 m, what will be the repulsive force?

1. 5 N

2. 10 N

3. 20 N

4. 40 N

5. 80 N

Example 2

What is the net force on the charge q due to the charges Q1

and Q2 placed as shown?

nCq 1

CQ 41

CQ 32

d

d

mmd 3

Example 2 (cont’d) What is the net force on q?

q

1Q

2Q

d

d

CQ

CQ

nCq

mmd

3

4

1

3

2

1

NN

d

qQkF

4104101

)103(

)101)(104(109

1515

23

969

2

11

NN

d

qQkF

3103101

)103(

)101)(103(109

1515

23

969

2

22

F=Fnet

F1

F2

NFFF

FFF

543222

2

2

1

21

37

4/3tan

Example 2 (cont’d)What is the net force on q?

NFx 4

q1Q

2Q

d

d

F

F1

F2

OR:y

x

NFy 3